In many modern wireless communications systems, information is organized into data units. When transmitted, the data units may be partitioned into transmission packets, with the number of packets depending on the size of the data units. The data units contain the information being transmitted along with control information. The control information includes destination information, network identifier, data rate, information length, and the like. For example, in an IEEE 802.11a wireless network, each data unit begins with a 16 micro-second field containing a short and a long sequence field, with each field being eight micro-seconds in length. The short and the long descriptors refer to the periodicity of the sequences. The 16 micro-second field contains ten periods of the short sequence and two and a half periods of the long sequence. Following the 16 micro-second field is another field containing information such as the bit-rate and the encoding of the data that is to follow.
The short sequence is used mainly to allow the wireless stations to detect the presence of a transmitted packet (which in turn, contains at least a portion of the data unit) on the shared communications medium and to adjust its receiver signal gain to bring the received signal to a level acceptable for processing purposes. The purpose of the long sequence is to allow the intended recipient of the data unit to make adjustments to its receiver hardware to maximize the probability of accurately receiving the data unit. The adjustments include configuring the receiver's adaptive channel equalizer and digital filters to current communications channel conditions.
In many communications systems, training sequences are typically transmitted concatenated together, without any indicator (or boundary) of when one sequence ends and when another begins. For sequences that are used to adjust receiver hardware and software, it is vital that the particular sequences be recognized as rapidly as possible.
A proposed solution involves correlating the received signal with a locally stored copy of the desired signal or, in the case of when the desired signal is periodic and the transmission contains several periods of the desired signal, correlated with previously received signals. Whenever the correlation results in a correlation value that exceeds a predetermined threshold, the desired signal is deemed to have been received and when the correlation value drops off, a boundary between different sequences is detected. Classically, correlating two signals involves multiplying one by the complex conjugate of the other. When limited precision is used, this is equivalent to comparing individual data values from each sequence and if the data values match, a correlation value is incremented. However, comparing pairs of data values from the received signal requires that a significant percentage of the different signal be received before the presence of the change can be detected.
A need has therefore arisen for a method to provide rapid determination of boundary between different sequences in a digital data stream and that can also be used to detect the presence of a transmitted packet in a formerly idle communications medium.